Put to the test

Understanding the nondestructive weld testing techniques available to fabricators

Nondestructive weld testing (NDT) is the process of inspecting, testing and evaluating welds to determine whether there are any flaws or defects without destroying any part of the test piece. In many industries, it’s crucial to confirm that a weld adheres to welding standards by examining the weld and the surrounding metal. NDT is regularly used in manufacturing and fabricating to ensure the reliability of systems and products.

Some of the techniques used in NDT are relatively straightforward while others require specially designed equipment. There are pros and cons to the different methods, ranging from cost – some are more expensive than others – to the various approaches that can be applied. Unsurprisingly, different methods can focus on identifying different types of flaws.

During a visual inspection, inadequate penetration, undercut and porosity are some of the things to look out for.

Seeing is believing

Visual inspection is often considered the most cost-effective NDT technique because it doesn’t require any equipment whatsoever. In fact, visual inspection is required as a prerequisite for other NDT examinations. A weld with more obvious defects can easily be assessed by visual inspection, saving the need for special equipment to be moved on-site when it may not be needed.

Visual inspection needs to take place before, during and after the weld to ensure welding standards are met. Before the weld, the metals should be thoroughly examined. The metal should be clean and flat and the dimensions should be accurate with the correct alignment.

During the weld, visual inspection should take place by observing the weld bead. Inadequate penetration, undercut and porosity are some of the defects to observe. After welding, inspect the surface of the weld for any flaws. These include anything from warping of the metal, size of the weld and cracks to unfilled craters and porosity.

Liquid dye penetrant testing involves applying a dye, which often comes in an aerosol can. During the NDT, the dye will absorb into the weld to reveal defects.

It’s in the dye

Liquid dye penetrant is a relatively simple NDT to perform that highlights any holes or cracks in the weld. One of the benefits of this method is that it doesn’t require a lot of expensive equipment. To perform the test, the following procedure must be followed.

1. Start by cleaning the weld, including removing any silica from the weld.

2. Once the joint is clean, apply the dye to the weld. This typically comes as a red spray in an aerosol can. Apply quite a lot onto the weld to ensure it penetrates sufficiently.

3. Leave the dye on for about a half an hour, keeping in mind that the correct time scale varies depending on the equipment used. After this period, spray cleaner around the weld to remove any excess dye.

4. Once the weld is clean, spray white developer onto the weld to draw out any dye that has been absorbed by the weld defects. These appear as color on the developer.

This is the most popular technique of dye penetrant testing, but alternatively, a fluorescent penetrant instead of a visible penetrant can be used, which tends to provide better results. When this method is used, an ultraviolet light is needed to expose the dye, which glows if there is a defect.

Below the surface

Radiographic inspection is a thorough and reliable method of inspection, best suited to detect defects inside the weld. It is performed by placing an X-ray film on one side of the joint and an X-ray tube on the other side.

A fluorescent penetrant can also be used in NDT, which glows if there is a defect within the weld.

During the radiographic inspection, X-rays are emitted into the joint, resulting in the film showing a black and white image that highlights any weld defects. Defects can include any cracks that are hidden within the joint.

The human eye can identify these defects on the film, but there is also software available to highlight the defects to help rule out any that might be missed by human error. It’s important to know that this type of NDT can be an expensive technique, particularly if the radiographic equipment has to be moved on-site. It’s mainly used on equipment where weld strength is critical to safety.

Ultrasonic inspection is another NDT that is used to detect discontinuities inside the weld. It is often used as a means of testing thick sections of metal that are accessible from only one angle.

Ultrasonic inspection is performed by firing ultrasonic waves into the base metal and the weld. If there is a discontinuity in the weld, the wave’s path is interrupted and it will bounce back to its origin. The sound waves are then displayed on a screen and are ready for interpretation.

Interpreting these results requires experience, skill and training to identify whether there are defects within the weld.

During ultrasonic NDT, ultrasonic waves are fired into the base metal and weld. If there is a discontinuity in the weld, the wave’s path is interrupted and it will bounce back to its origin.

Magnetic attraction

Magnetic particle inspection is used for detecting deficiencies in magnetic metals. It is primarily used for identifying surface or marginally subsurface flaws, such as cracks that are too small to be seen by the human eye.

In this process, the ferromagnetic material needs to be magnetized by passing high amperage through the metal. A magnetic flux is then produced through the weld, and if the weld does not have any defects, the flux will stay inside the metal.

However, if there is a crack in the weld, the magnetic flux will leak through the defect. This magnetism can then be displayed by placing fine iron particles or powder around the weld that will accumulate over the crack where the magnetism is leaking. This powder is available in different colors to make the cracks easy to see and is also available as a fluorescent powder that is illuminated under ultraviolet light.

To better understand which NDT methods are best-suited for various welding operations, it’s recommended to work with someone who has been certified in NDT. The American Welding Society as well as the American Society for Nondestructive Testing are both good resources for finding certified personnel and related information.